The invention relates to heat-hardenable reaction resin mixtures which contain polyfunctional epoxides, polyfunctional isocyanates and a reaction accelerator as well as optional additives and/or fillers. The invention as well relates to the use of such reaction resin mixtures.
In electrotechnology, epoxy resins and in particular, acid-anhydride-hardenable epoxy resins are important because, being reaction resins which are employed without solvents, they can be used for many applications, especially for insulating purposes. The epoxy resin molded materials made from the epoxy resins exhibit very good thermal-mechanical properties. The temperature range in which they must be operative, however, includes both very high and very low temperatures. This requires epoxy resin molded materials with improved temperature cycle resistance and at the same time, high dimensional heat stability.
It is known that the temperature cycle resistance of acid-anhydride-hardened epoxy resin molded materials can be improved by the addition of flexibilizers (see, for instance, Henry Lee and Kris Neville, "Handbook of Epoxy Resins", McGraw-Hill Book Company, 1967, pages 16-1 ff). Frequently used flexibilizers are, for instance, polyoxyalkylene glycols as well as polyoxyalkylene glycol polyglycidyl ethers which are incorporated by hydroxy or epoxy functions into the polymer network. The improvement of the temperature cycle resistance by flexibilizing with polyoxyalkylene glycol polyglycidyl ethers, however, causes a drop in the required high dimensional heat stability. Since, in addition, acid-anhydride hardenable epoxy resins are not very suitable for the design of insulating materials with a fatigue strength of more than 155.degree. C., new reaction resins with high dimensional heat and hot-fatigue stability and at the same time good mechanical properties and good temperature cycle resistance must be developed. Polyimide resins and polyamide imide resins as well as bismaleimide resins would in principle be suitable therefor. These resins are eliminated, however, as they are difficult to prepare as low-viscosity resins without solvents. Silicones are not suited because they have insufficient mechanical properties, especially at high temperature, and have insufficient adhesion to metal surfaces.
Molded materials which are built up from isocyanurate heterocycle structures (hereinafter ICR), have been found to be particularly heat resistant. These heat-resistant heterocycles as well as heat-resistant oxazolidinone structures (OX) are generated in the hardening of reaction resin mixtures of polyepoxides and polyisocyanates (hereinafter EP/IC resins). The molded materials (e.g., OX/ICR molded materials), which can be produced from the EP/IC resins and which contain OX and ICR rings, exhibit high dimensional heat stability or resistance. The dimensional heat resistance of these molded materials increases with increasing content of isocyanurate rings. At the same time, however, also the brittleness of the resetting molded materials increases, i.e., the mechanical properties and the temperature cycle resistance become worse. If EP/IC resins are used for the manufacture of highly heat-resistant insulating materials, the temperature cycle resistance and the crack resistance, respectively, of the OX/ICR molded materials are found to be too low. When they are used as coating resins for metallic conductors, the tendency of the OX/ICR molded materials to develop cracks is additionally increased by the large difference of the linear thermal coefficient of expansion between the metal and the resin. If it is possible on account of the boundary conditions, this difference can be reduced by adding mineral fillers, but the usable filler content is limited by the increasing viscosity of the resin mixture as well as by the degradation of the mechanical properties. Therefore, there is a demand for highly heat-resistant polymer molded materials which have at the same time an excellent continuous temperature cycle resistance and good mechanical properties.
Therefore, it is an object of the invention to develop heat-hardenable reaction resin mixtures based upon polyfunctional epoxides and polyfunctional isocyanates which permit the manufacture of highly heat-resistant molded materials and which have at the same time high temperature-cycle resistance and good mechanical properties.